STRETCHING OF THE STRINGS
New music wire has a lot of elasticity, and begins to stretch as soon as you pull it When you put new strings on a piano, they're pretty stretchy. In fact, they start stretching as soon as you tune them to the right pitch. This stretching is most noticeable during the first few years. So, if you have a new piano, you should get it tuned at least three or four times in the first year. After that, the strings stretch less, and the pitch stays steady for longer. But don't forget, you should still get your piano tuned at least twice a year to keep it in good shape. Following this schedule will help you manage the cost of piano tuning.
SLIPPING TUNING PINS
When pianos are exposed to regular changes in humidity, the pins that hold the strings in place can start to slip. When these pins get looser, the string tension can slowly make them turn over several months, causing the pitch to go flat.
The more you play your piano, especially if you play loudly and often, the faster it's going to go out of tune. This happens because when you play, the tension along the strings evens out, and that can affect with the tuning.